render only composite
Delete the renderlayer node from the node setup, and then blender won't render the 3D scene.

If there is anything in the VSE or the compositor,  by default blender will prioritize rendering it over rendering the scene unless you explicitly disable it in the render settings:



If use nodes is enabled, then blender will only render the 3D if it's needed for the composite. (i.e. there is a renderlayer node using the scene as input)